Geology in space

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Winter in the northern hemisphere is synonymous with snow (at least in films and Victorian novels). The Earth is unusual because it has water can commonly be found on its surface in solid, liquid and gas form, and both solid and liquid water can rain/snow/sleet/drizzle from the sky. So what happens on other bodies in the solar system?

Clouds of ice crystals on Mars – Ready to snow? (NASA/JPL)

Most water on the surface of Mars is now trapped in its glaciers/ice caps, but what about snow? Whilst there is less water vapour in the Martian atmosphere than the Earth’s, Mars still has clouds made of ice crystals. These ice crystals can fall slowly out of the sky, but at night under the right conditions, snowstorms can occur. Most of this snow will vaporise before it hits the ground, and any that do make it to the ground will likely vaporise during the day.

Maxwell Montes the highest mountain on Venus, appears brighter than the surroundings because of sulphide frost (Image NASA/JPL)

Venus has the hottest surface temperature of any planet in the solar system. Water snow is not possible. The atmosphere of Venus contains lots of sulphur and temperatures hot enough to melt lead, it is so thick we can’t see the surface of the planet so we have collected radar data to understand what it looks like. One feature of this data is that on the tops of some high mountains there is an increase in the number of radio waves reflected, giving the appearance that the tops of mountains are brighter, just as snow on the top of mountains is brighter than the surrounding rock.

Whilst it is far too hot for snow on Venus, it is thought that these mountain tops are covered in a form of frost caused by metal sulphides, which forms only at high elevations and it’s this which makes is appear more reflective to radar.

Mercury, my main planet of study, doesn’t have snow-capped peaks, being too hot over the majority of the planet and no atmosphere. So the nearest feature to snow is ice in the craters close to the north and south poles, these craters are permanently in shadow protecting the ice from the boiling Sun. Without an atmosphere there isn’t any form of precipitation other than a daily micrometeorite shower.

The south pole of Mercury, the black areas are permanently in shadow and thought to contain water ice. (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Titan, a moon of Saturn is much to far out from the Sun to have water snow, and any water near the surface, just like in the bleak midwinter, is like a stone. It does snow on Titan, near the poles hydrocarbons can crystallize out in the atmosphere and then fall as snow down onto the surface.

Another moon of Saturn, Enceladus, is probably the most visually spectacular example, geysers on the south pole can be seen erupting water vapour into space. Some of this water is lost in orbit and forming the E ring of Saturn, but some of the water falls back down onto the surface as ice crystals. As this fall back it is happening without an atmosphere it doesn’t really count as snow but is a great excuse to show the stunning photos of the plumes.

Often during Halloween ghost stories are told. Phantoms from the past linger in the present trying to tell us about some previous horror. We know that there are ghosts on Mercury.

Mercury is small, smaller than the moons Titan and Ganymede. Small bodies lose heat quickly and don’t generate as much internal heat as larger planets. That that is not to say that it was never hot, in the past Mercury would have been much hotter, and with that volcanically active. The planet’s surface has been covered over and reworked by volcanic activity often multiple times.

We see evidence of volcanoes on Mercury from Lava filled craters, and lava flows flowing out of them, bright spots on the surface which represent fire fountains and pyroclastic deposits. Smooth plains cover over much older crater marked surfaces.

Red spots in the middle of Derain crater, the crater has been filled in with lava giving it a smooth appearance, the red marking represent later pyroclastic eruptions (NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington)

Molten rock will follow weaknesses in the rock in order to find the easiest route, on Mercury, these are often found in impact craters, filling them up like bowls. As the bowls of lava cool, they contract the top starts to wrinkle, like the skin on custard. These wrinkles form long bands of hills imaginatively called wrinkle ridges.

In some places lava flows are very extensive forming planes which completely cover craters, concealing the scar left by an impact on the planet’s surface. because the lava is thinner at the rim of the crater and so cools and shrinks at different amounts to the lava surrounding this. The differential shrinking leads to the formation of either wrinkles or in some cases faults forming. The crater, completely buried by lava, becomes apparent, traced out on the surfacece of the lava. The outlines in the lava are known as ghost craters.

Since graduating from my masters I, have been working for a mining company in West Africa. I have had some great times meeting amazing people as well as seeing some amazing geology, at night, during in the dry season, I would get some awe-inspiring views of the rest of the universe.

My photo of Venus and the Milkyway take from Liberia in 2016

From an early age I was always fascinated with dinosaurs and volcanoes, so I tried to find out more about them, a terrible memory for names meant that the dinosaurs fell by the wayside, but my interest in volcanoes grew into wanting to understand how the Earth worked and slowly I turned into a geologist.

A rocky shoreline on Titan ESA/NASA/JPL/University of Arizona

At the same time, the rest of the universe has been there from staring at shooting stars to the Galileo mission, it’s photos of Jupiter and the moons were part of my childhood. In 2005 the photos Huygens probe touchdown on Titan opened my eyes to how surprisingly familiar alien worlds can be (maybe that says something about British seaside holidays).

So after 6 years as a geologist on Earth, I realised one planet was not enough, it’s time for a change of course. I have just started PhD at the Open University where I will be researching Mercury using Nasa’s MESSENGER data to make geological maps of part of the planet, contributing to our knowledge of the smallest planet in the solar system. During the coming weeks, I hope to publish some more specific blogs on Mercury as well as the more general planetary geology and from time to time I’ll post updates on my progress.

For the smallest of the planets, Mercury is a surprisingly active place with a complex internal structure and magnetic field, however, it’s tectonics is dominated by its size.

Smaller bodies lose heat a lot faster than bigger bodies, due to a higher ratio of surface area to volume, this means that Mercury has cooled relatively rapidly. As the planet cooled it has shrunk. Estimates for this contraction of its radius range from ~2 – 7 km over its lifetime, whilst this is only 0.2% of its total radius, this equates to its diameter shrinking 44 km at its equator.

Mercury’s surface is a single tectonic plate, it doesn’t have the subduction and rifting zones which can accommodate strain. As the planet has cooled and contracted the crust at the surface has become compressed as the larger diameter crust is pulled in to fit into a smaller area. Rocks when under compression buckle and eventually break and form faults. In the case of Mercury, shallow angle faults known as thrust faults are created. As one part of the crust rides over another it forms cliffs (called “Rupes”) which can be 100’s of km long and several hundred meters high.

Carnegie Rupes, 2 km high wall running diagonally across the image created by a thrust fault (NASA/Johns Hopkins University Applied Physis Laboratory/Carnegie Institution of Washington)

Where multiple faults interact they can produce complex structures. The Great Valley is 1000 km long valley formed from thrust faults which make up either side and has been bent downwards in the middle.

The Great Valley (in Blue) close to Rembrandt basin on Mercury, (Nasa/JHUAPL/CIW/DLR/SI)

Smaller scale scarps have also been identified, whilst smaller (10’s of km long and 10’s of meters high) they are often found cutting across small impact craters and smooth younger planes which means that these features are less than 50 million years old and suggests that Mercury is still tectonically active now.